Solutions prepared from alkaline earth metal or alkali metals (“M”) and aluminum in ethylene glycol tend to precipitate over time. This problem is especially noticeable at molar ratios of M:Al approaching 1:1, where precipitation begins to occur at moderate temperatures less than 125° C. This is because aluminum compounds do not easily dissolve in ethylene glycol. The feed of catalyst components to a melt phase polymerization process should stay in solution to provide more uniform mixing with the reactants or polymer melt, and to enable feeding a consistent and uniform amount of desired catalyst to the melt phase process.
A catalyst solution has advantages over catalyst slurries, in that a solution avoids the potential for pumping and circulation problems, avoids transfer line fouling and plugging, and avoids the need for vigorous agitation used in slurries to prevent insoluble catalyst precipitates from settling in feed tanks. Precipitates in the feed tanks make feeding a uniform amount of catalyst to a melt phase production line a problem.
Lithium hydroxide and aluminum isopropoxide can be combined in the presence of ethylene glycol to form a solution. This can be accomplished by heating the components to a temperature sufficient to form the catalyst in solution. The temperature for this reaction is normally in the range of 125° C. to 160° C. for three to five hours. Generally, the concentration of aluminum in the solution cannot exceed 3,000 ppm without a precipitate or gel forming upon cooling the mixture to ambient room temperatures.
Precipitates can form under several conditions when a catalyst system is mixed in ethylene glycol. Precipitates can form when the catalyst solution cools down to ambient temperatures. Even when the composition remains as a solution upon cooling, over time (e.g. a matter of a two or three days) the solution can change to form precipitates. The amount of actual catalyst fed to a melt phase polymerization line for making the polyester through a feed system set at a given flow rate will fluctuate if precipitates form, thereby leading to inconsistent product types or product quality.
To maintain the catalysts in solution, an ethylene glycol/Li/Al catalyst composition must remain at an elevated temperature of about 150° C. or more. Catalyst solutions maintained at high temperatures suffer from several disadvantages. Catalysts held for extended periods of time at elevated temperature can potentially lead to catalyst deactivation. Further, to maintain the catalyst solution at elevated temperatures requires increased plant capital for heated catalyst feed vessels.
Another way precipitates form is when the amount of aluminum in the catalyst composition exceeds 3000 ppm. While it is desirable to employ a catalyst feed source having a high concentration of Al so that the amount of solvent fed to the melt phase process can be reduced, it is necessary to maintain the catalyst in solution which becomes more difficult as the amount of aluminum increases.
Not only can the catalyst precipitate in ethylene glycol solutions 3000 ppm aluminum or more is used or if the hot solution is allowed to cool, but it can precipitate as the molar ratio of M:Al approaches 1:1. However, a molar ratio of M:Al of about 1:1 is desirable in some applications because the yellowness of the polyester polymer is minimized as the molar ratio of M:Al approaches 1:1.
Adding a molar excess of hydroxyacid to the metal M is undesirable because the cost of the solution is increased. It is desirable to maintain the flexibility to add stoichiometric amounts of M to the hydroxyacid and even a molar excess of M to hydroxyacid while retaining a solution having low quantities of hydroxyacid compounds. The catalyst composition desirably exhibits increased solubility in ethylene glycol relative to a solution of ethylene glycol as the sole solvent, advantageously with minor amounts of hydroxyacid and a large stoichiometric excess of ethylene glycol relative to the hydroxyacid compounds. Alternatively, or in addition, it would also be desirable if solutions can be made, if desired, at molar ratios of M:Al that approach 1:1 that are stable over a wide variety of temperatures, including ambient conditions. Alternatively, or in addition, it would be particularly advantageous if such solutions can be made using 3000 ppm Al or more to minimize the amount of solvent fed to a melt phase polycondensation process.